Abstract: Cationic polyurethanes and a process for their production wherein a polyisocyanate, preferably a diisocyanate such as 2,4- or 2,6-tolylene-diisocyanate is reacted with an aliphatic dihydroxy compound substituted with an aliphatic side-chain of at least 10 carbon atoms to form a preliminary adduct or so-called prepolymer with terminal -- NCO groups. This prepolymer is then reacted for chain-lengthening with an aliphatic monomeric diol (A) containing a tertiary nitrogen atom convertible into an ammonium compound or an aliphatic monomeric diol (B) containing a tertiary nitrogen atom already converted into an ammonium compound. The new cationic polyurethanes are especially useful as sizing agents for the sizing of paper.